1. Field of the Invention
The present invention relates to formation of silicon-germanium alloys and, in particular, to the formation of silicon-germanium alloys utilizing energy from an applied laser.
2. Description of the Related Art
Silicon-germanium alloys are finding increased use in semiconductor manufacturing due to the emergence of band gap engineering to control conductance. Silicon-germanium alloys are particularly useful for forming the base material of high speed heterojunction bipolar transistor (HBT) devices.
FIGS. 1A-1F show a conventional process flow for forming an HBT.
FIG. 1A shows the starting point for the process, wherein single crystal silicon workpiece 100 containing dopant of a first conductivity type is exposed to an ambient containing a mixture of silane (SiH4) 102 and germine (GeH4) 104 gases. FIG. 1B shows the resulting deposition of silicon-germanium alloy 106 over the surface of single crystal silicon workpiece 100. FIG. 1C shows the ion-implantation of dopant 108 of a second conductivity type opposite the first conductivity type into silicon-germanium alloy 106. FIG. 1D shows the subsequent formation of polysilicon layer 110 over the doped silicon-germanium alloy 106. Polysilicon layer 110 is then highly doped by ion-implantation of dopant 112 of the first conductivity type, as shown by FIG. 1E. FIG. 1F shows completion of fabrication of HBT structure 114, wherein polysilicon layer 110 and silicon-germanium alloy layer 106 are selectively removed to reveal polysilicon emitter 116 overlying and separated from single crystal silicon collector 118 by silicon-germanium base 120.
While the above FIGS. 1C and 1E depict introduction of conductivity-altering dopant by ion-implantation, it is also well known in the art to introduce dopant by chemical vapor deposition followed by thermal drive-in.
While satisfactory for some applications, the conventional process for forming the HBT suffers from a number of disadvantages. In particular, formation of the silicon-germanium alloy of the base by co-deposition of silicon and germanium-containing gases, as shown in FIGS. 1A-1B, produces an alloy having uneven concentrations of silicon and germanium. In addition, the conventional co-deposition technique produces a silicon-germanium alloy of relatively low crystal quality. Both the uneven Sixe2x80x94Ge concentration and the poor crystal structure of the conventionally-formed alloy degrade the operational characteristics of the HBT.
Therefore, there is a need in the art for a process for forming a high quality silicon-germanium alloy having uniform silicon and germanium concentrations.
The present invention relates to a process for forming a high quality crystalline doped silicon-germanium alloy utilizing, laser annealing. An amorphous or polycrystalline doped germanium film is first formed over epitaxial silicon. Application of radiation from a laser beam to the amorphous/polycrystalline doped germanium layer melts both the germanium layer and a portion of the underlying epitaxial silicon. The high temperature of melting promotes diffusion of both germanium and dopant into the underlying silicon. Removal of the laser beam causes the silicon-germanium-dopant melt to cool and recrystallize in high quality crystalline form. Diffusion of germanium and dopant during the melting step ensures uniform incorporation of these materials into the silicon-germanium lattice.
A first embodiment of a process for forming a silicon-germanium alloy in accordance with the present invention comprises the steps of forming a doped amorphous/polycrystalline germanium layer over a single crystal silicon workpiece. A laser beam is applied to melt the doped polycrystalline/amorphous germanium layer and single crystal silicon and cause dopant from the doped germanium layer to diffuse into the silicon. Removing the laser beam causes melted germanium and silicon to solidify to form a silicon-germanium alloy incorporating dopant from the amorphous/polycrystalline germanium layer.